Superhydrophobic surfaces (i.e., surfaces with water contact angles higher than 150°) have drawn great scientific and industrial interest due to their applications involving water repellency and self-cleaning and their anti-fouling properties. Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857; Quéré, D. Nature Mater. 2002, 1, 14; Lafuma, A.; Quéré, D. Nature Mater. 2003, 2, 457; Blossey, R. Nature Mater. 2003, 2, 301; and Erbil, H. Y.; Demirel, A. L.; Avc1, Y.; Mert, O. Science 2003, 299, 1377. Generally, both surface chemistry and surface roughness affect hydrophobicity. Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31; and Quéré, D. Physica A 2002, 313, 32. For a flat solid surface, the contact angle (θ) can be described by Young's Equation: cos θ=(γSV−γSL)/γLV, where γij is the surface tension of the solid-vapor, solid-liquid and liquid-vapor interfaces, respectively. Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. However, due to the limitations of interfacial tension, surface chemistry alone is insufficient to achieve superhydrophobicity. A superhydrophobic surface requires in addition a certain surface roughness. Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988; Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466; Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546; Johnson, R. E.; Dettre, R. H. Adv. Chem. Ser. 1964, 43, 112; and Johnson, R. E.; Dettre, R. H. Adv. Chem. Ser. 1964, 43, 136. In the Wenzel hydrophobic state, the water droplet penetrates into the surface cavities and remains pinned to the surface, which magnifies the wetting property of the surface and leads to a high hysteresis (the difference between the advancing and the receding contact angles) or a high threshold sliding angle. In this state, the surface roughness (r), defined as the ratio of the actual contact area to the apparent surface area, is used to relate the apparent contact angle (θ*) and θ, as cos θ*=r cos θ. In the Cassie-Baxter state, the liquid does not follow the surface contours, but bridges across the surface protrusions and sits upon a composite surface composed of both solid and air patches; in this case, the apparent liquid contact angle is described by cos θ*=φs cos θ−φv, where φs and φv are the solid-liquid and gas-liquid contact area per unit projected surface area, respectively. The Cassie-Baxter state also has a low hysteresis and threshold sliding angle because the water can slide or roll easily when it sits partly on air; therefore, in real applications, the stable Cassie-Baxter state is generally more desirable for applications where water needs to be shed. Based on these principles, numerous methods have been reported to produce superhydrophobic surfaces by either increasing the surface roughness of an inherently hydrophobic material or decreasing the surface free energy of a rough surface by post treatment. Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125; Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y; Jiang, L.; Zhu, D. Angew. Chem. Int. Ed. 2002, 41, 1221; Quéré, D.; Lafuma, A.; Bico, J. Nanotechnology 2003, 14, 1109; Yoshimitsu, Z.; Nakajima, A.; Watanabe, T.; Hashimoto, K. Langmuir 2002, 18, 5818; Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701; Woodward, I.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432; and Zhai, L.; Cebeci, F. C.; Cohen, R. E.; Rubner, M. F. Nano Lett. 2004, 4, 1349.
Electrospinning is a simple but versatile method to produce continuous, submicron diameter fibers. It has recently been shown to provide the appropriate surface roughness to make superhydrophobic surfaces. Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem. Int. Ed. 2004, 43, 4338; Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem. Int. Ed. 2004, 43, 5210; and Ma, M.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir, 2005, 21, 5549. Both experimental and theoretical studies have been conducted to characterize the process and control the fiber morphology. Fong, H.; Chun, I.; Reneker, D. H. Polymer 1999, 40, 4585; Reneker, D. H.; Yarin, A. L.; Fong, H.; Koombhongse, S. J. Appl. Phys. 2000, 87, 4531; Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Polymer 2001, 42, 9955; Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. C. Appl. Phys. Lett. 2001, 78, 1149; Theron, S. A.; Zussman, E.; Yarin, A. L. Polymer 2004, 45, 2017; Yarin, A. L.; Koombhongse, S.; Reneker, D. H. J. Appl. Phys. 2001, 89, 3018; Yarin, A. L.; Koombhongse, S.; Reneker, D. H. J. Appl. Phys. 2001, 90, 4836; Hohman, M. M.; Shin, Y. M.; Rutledge, G. C.; Brenner, M. P. Phys. Fluids 2001, 13, 2201; Hohman, M. M.; Shin, Y. M.; Rutledge, G. C.; Brenner, M. P. Phys. Fluids 2001, 13, 2221; Feng, J. J. Phys. Fluids 2002, 14, 3912; and Fridrikh, S. V.; Yu, J. H.; Brenner, M. P.; Rutledge, G. C. Phys. Rev. Lett. 2003, 90, 144502. These studies clearly show that the formation of ultrathin fibers is achieved by the stretching of the polymer jet associated with the onset of a whipping instability caused by the electrostatic forces. The polymeric fluid must have adequate viscoelasticity (usually controlled by an appropriate combination of molecular weight and concentration of the polymer in solution) and conductivity in order to be electrospun, i.e., to form uniform fibers. Otherwise, the surface tension, which tends to break the liquid jet into droplets (the effect known as Rayleigh instability), dominates the process, and beaded fibers or polymeric microdroplets will be formed instead of uniform fibers. The diversity of electrospun materials and the interesting properties of electrospun fibers have led to applications ranging from composite materials, sensing technology, and filtration to tissue engineering and biomedical applications. Frenot, A.; Chronakis, I. S.; Curr. Opin. Colloid Interface Sci. 2003, 8, 64; Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S. Compos. Sci. Technol. 2003, 63, 2223; Li, D.; Xia, Y. Adv. Mater. 2004, 16, 1151; Dzenis, Y. Science 2004, 304, 1917; and Thandavamoorthy Subbiah; Bhat, G. S.; Tock, R. W.; Parameswaran, S.; Ramkumar, S. S. J. Appl. Polym. Sci. 2005, 96, 557.
Initiated chemical vapor deposition (iCVD) is a one-step, solvent-free deposition technique. The conformal nature of the iCVD process enables coating on complex substrates. It allows films of nanoscale thicknesses to be produced and has been used to coat nanoscale features. Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; McKinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. Fluoropolymer coatings are well known for their low surface energies, with poly(tetrafluoroethylene) (PTFE, (—CF2—)n) having γs of about 20 mN/m and fluorinated acrylic polymers exhibiting even lower values of γs (about 5.6 to about 7.8 mN/m) due to their CF3 terminated side chains and their comb-like structures. Thunemann, A. F.; Lieske, A.; Paulke, B. R. Adv. Mater. 1999, 11, 321; Anton, D. Adv. Mater. 1998, 10, 1197; and Tsibouklis, J.; Nevell, T. G. Adv. Mater. 2003, 15, 647. The iCVD technique has been successfully applied in polymerizing perfluoroalkyl ethyl methacrylate (PFEMA, CH2═C(CH3)COOCH2CH2(CF2)nCF3, n˜7, Zonyl®) using tert-butyl peroxide as an initiator (results not shown). The dispersive surface energy of the resulting poly(PFEMA) (PPFEMA) coating is 9.3 mN/m. The process involves thermal decomposition of the initiator molecule into free radical species and subsequent addition reaction of the monomer, as shown in FIG. 1.